This invention relates to a process for the alcoholysis of ethylene-vinyl ester interpolymers, and more particularly to such a process for controlling the alcoholysis to produce partially hydrolyzed materials having precise, predetermined residual vinyl ester contents.
Processes for hydrolyzing ethylene-vinyl ester interpolymers by alcoholysis in hydrocarbon solvent media, utilizing a wide variety of catalysts, are well known in the art. Such procedures are described, for example, in Roland U.S. Pat. No. 2,386,347, granted Oct. 9, 1945; Sharkey U.S. Pat. No. 2,396,210, granted Mar. 5, 1946; Plambeck U.S. Pat. No. 2,467,774, granted Apr. 19, 1949; Bryant et al U.S. Pat. No. 2,668,809, granted Feb. 9, 1954; Bestian et al U.S. Pat. No. 3,344,129, granted Sept. 26, 1967; Salyer U.S. Pat. No. 3,386,978, granted June 4, 1968; and Bristol U.S. Pat. No. 3,510,463, granted May 5, 1970.
It is recognized in the art that it is difficult to achieve sufficient control over the degree of hydrolysis of ethylene-vinyl ester interpolymers to prepare products of uniform composition. For example, in Roland, U.S. Pat. No. 2,399,653, granted on May 7, 1946, it is disclosed that, in saponification with an alkaline reagent such as an alkali metal hydroxide in solution in a primary alcohol, the hydrolysis is much greater than that which may be accounted for on the basis of reaction of the ester group with the base. Roland sought to overcome this problem and obtain the desired predetermined partial hydrolysis by substantially eliminating the primary alcohol and conducting the reaction in the presence of at least 10% by weight of a secondary or tertiary alcohol incorporating from 3 to 6 carbon atoms. More recently, however, Salyer (U.S. Pat. No. 3,386,978) has disclosed that catalytic hydrolysis of ethylene-vinyl acetate copolymers, for example, can be controlled in the presence of primary alcohols, but that a unique, 1:1:1 mixture of toluene, ethanol, and methanol must be used as the reaction medium for this purpose.
Neither Roland nor Salyer recognized any relationship between reproducibility in the degree of partial hydrolysis of the base resins and the aggregate initial water content of their components, determined prior to combining the components in the reaction zone to form the alcoholysis reaction mixture.
It is also indicated in the art that ethylene-vinyl ester copolymers containing relatively small proportions of the vinyl ester are difficult to hydrolyze because of their poor solubility. Thus, Bestian (U.S. Pat. No. 3,344,129) teaches that such copolymers are hydrolyzed very slowly by alcoholysis in methanol or ethanol as a result of their poor solubility in these solvents. Non-uniform reaction products are said to be obtained therefrom, containing both hydrolyzed and unhydrolyzed molecules. According to Bestian, the previously proposed use of mixtures of aromatic hydrocarbons with alcohols as reaction solvents (Roland U.S. Pat. No. 2,386,347) is only successful in the hydrolysis of those ethylene-vinyl ester polymers in which the molar proportion of vinyl ester to ethylene is greater than 1:5, i.e., in the case of ethylene-vinyl acetate copolymers, copolymers incorporating more than 38% of vinyl acetate by weight. Bestian indicates that when polymers having lower contents of vinyl acetate are hydrolyzed the amount of aromatic hydrocarbon solvent required is quiet high and thus economically disadvantageous, and the speed of reaction is so slow that reaction practically comes to a standstill. To overcome these deficiencies, Bestian suggests the use as reaction solvents of high alcohols having from 4 to 8 carbon atoms, wherein improved solubility may be effected by operating at higher reaction temperatures.
It has also been claimed that small amounts of water in the reaction medium may have deleterious effects on the reaction. British Pat. No. 1,120,189 teaches that, in the alcoholysis of ethylene-vinyl acetate copolymers in the presence of an alkali metal alcoholate catalyst, traces of water cause the catalyst to react with acetic acid ester by-product. To overcome this difficulty it is proposed in the patent to continuously pass an alcohol vapor stream through the reaction zone to sweep out the acetate ester as soon as it is formed. On the other hand, Bristol (U.S. Pat. No. 3,510,463) notes that in the alcoholysis of pelleted ethylene-vinyl ester copolymers, traces of water may be present; this reference does not provide any teaching as to what effect, if any, small amounts of water may have on the reaction.
The alcoholysis of poly(vinyl acetate) itself is carried out in anhydrous methanol containing a maximum of a few percent of water [German Pat. Nos. 642,531 and 763,840]. However, it is also claimed that water has little effect on base-catalyzed alcoholysis of poly(vinyl acetate) [Sakaguchi et at, Kabunshi Kagaku, 20, 286 (1963), cited in "Ethylene and Its Industrial Derivatives", Ernest Benn, Ltd., London, 1969, p. 1025].
When ethylene-vinyl ester copolymers are hydrolyzed by alcoholysis, it is well known to those skilled in the art that the hydrolyzed products are often discolored. Thus, yellow-orange discoloration is very apparent in copolymers which are extensively hydrolyzed in the pelleted form (e.g., as described in the aforesaid Bristol U.S. Pat. No. 3,510,463). Such is confirmed in the noted British Pat. No. 1,120,189 in which it is noted that, unless special measures are taken, product discoloration is encountered in conventional industrial hydrolysis operations when the residual acetyl content is less than 10 mole percent.
It is also claimed in the art that hydrolyzed ethylene-vinyl acetate copolymers are sometimes seriously contaminated with inorganic salts produced from the alkaline alcoholysis catalysts. Salyer (U.S. Pat. No. 3,386,978) teaches that, when hydrolyzed copolymer in an aromatic solvent is recovered by precipitation with an alkanol, the inorganic substances present are not separated but must be removed by extensive washing with water. The relative amounts of inorganic residues left in the hydrolyzed copolymers and the significance of these residues is not indicated.
It is among the objects of the present invention to provide an improved process for the alcoholysis of ethylene-vinyl ester interpolymers resulting in the formation of partially hydrolyzed materials having predetermined, controllable residual vinyl ester contents. The products thus formed may be obtained with good purities, containing low levels of inorganic catalyst residues and being substantially free of discoloration. Other objects and advantages of the process of the present invention will be apparent from consideration of the following detailed description of preferred embodiments thereof.